1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus, and a method for controlling the same, and an image pickup apparatus.
2. Description of the Related Art
Conventionally, many methods for expanding a dynamic range of an image pickup device have been proposed.
Japanese Patent Application Laid-Open No. H11-313257 discloses a technique for outputting a signal corresponding to the logarithm of light incident on a photodiode (hereinafter referred to as “PD”).
Japanese Patent Application Laid-Open No. 2000-059688 discloses a technique for expanding the dynamic range by performing photoelectric conversion in both a PD and a floating diffusion (hereinafter referred to as “FD”).
Japanese Patent Application Laid-Open No. 2001-177775 discloses a technique for expanding the dynamic range by transferring charges generated in a PD to a FD a plurality of times.
In an image pickup device, when signal charges accumulated in the PD exceeds a saturation signal amount, the signal charges may leak into an area having a low potential barrier. The leaking state is shown in FIGS. 18A and 18B. In FIGS. 18A and 18B, the upper part illustrates a cross-sectional constitution of each unit, and the lower part illustrates a potential distribution of the each unit. The vertical axis of the potential distribution indicates the surface potential of a semiconductor substrate under an insulation layer. A lateral position in the potential distribution illustrated in the lower part corresponds to the lateral position in the cross-sectional constitution illustrated in the upper part. FIGS. 3A to 3D, FIGS. 5A to 5D and FIGS. 19A to 19D as will be described below are also similarly illustrated. As illustrated in FIG. 18A, signal charges generated in the PD is accumulated in a parasitic capacitance of the PD. When the signal charges accumulated in the parasitic capacitance of the PD exceeds a saturation signal amount, the signal charges exceeds the potential barrier of a transfer switch TX to leak into the FD, as illustrated in FIG. 18B.
Japanese Patent Application Laid-Open No. 2003-087665 discloses a technique for expanding the dynamic range by adding the signal charges leaking into the FD to signal charges generated in the PD by utilizing the phenomenon as illustrated in FIGS. 18A and 18B.
Further, in an image pickup device in recent years, in order to reduce the size of a pixel, attempts have been made to share a circuit (signal reading circuit) arranged in the pixel and used to read out signals of a PD, among a plurality of PDs to narrow the pixel pitch.
However, in the pixel structure sharing the signal reading circuit, it is very difficult to apply the method in which the dynamic range is expanded by trapping in the FD the excess charges generated in the PD. In the following, such a situation will be described with reference to FIGS. 19A to 19D and FIG. 20.
FIGS. 19A to 19D are figures in which the upper part illustrates a cross-sectional constitution of each unit of the pixel structure, and the lower part illustrates the potential distribution of the each unit. FIG. 20 is a figure illustrating a relation between an accumulated charge amount of each unit and an exposure time. In FIG. 20, the horizontal axis represents time and the vertical axis represents the charge amount. FIG. 19A corresponds to a state of the pixel at time point t51 in FIG. 20. FIG. 19B corresponds to a state of the pixel at time point t52 in FIG. 20. FIG. 19C corresponds to a state of the pixel at time point t53 in FIG. 20. FIG. 19D corresponds to a state of the pixel at time point t54 in FIG. 20. Further, FIG. 20 respectively illustrates a relation between an accumulated charge amount and an exposure time in a PD1, a relation between an accumulated charge amount and an exposure time in PD2, and a relation between an accumulated charge amount and an exposure time in an FD. Here, among the two PDs sharing a signal reading circuit, a PD which is read first is referred to as the PD1, and a PD which is subsequently read is referred to as the PD2. Further, it is assumed that the PD1 is a pixel provided with a color filter having a higher sensitivity as compared with the PD2.
At time point t51 (corresponding to FIG. 19A), the PD1, the PD2 and the FD are reset so that signal charges start to be accumulated.
At time point t52 (corresponding to FIG. 19B), the signal charges are accumulated in the PD1 and the PD2.
At time point t53 (corresponding to FIG. 19C), the accumulated charge amount of the PD1 exceeds a saturation charge amount. In the PD1, the potential barrier on the side of the transfer switch TX1 is lowest, and hence the signal charges generated in the PD1 start to leak into the FD.
At time point t54 (corresponding to FIG. 19D), the accumulated charge amount exceeds the saturation charge amount in the PD1 and the PD2. The potential barrier on the side of a transfer switch TX2 is lowest in the PD2. Therefore, the excessive charges exceeding the saturation charge amount in the PD1 and the excessive charges exceeding the saturation charge amount in the PD2 both start to leak into the FD, and the signal charges of the PD1 and the PD2 are intermixed in the FD.
As described above, when the signal charges leak into the FD from the plurality of pixels sharing the FD, the signal charges leaking from the PDs of the both pixels are intermixed. Therefore, in an image pickup device in which color filters are regularly arranged, information on an adjacent pixel having another color filter is intermixed, so that a signal which is different from the signal to be originally obtained, is generated. As a result, in the image pickup apparatus having the pixel structure sharing the FD, it has been very difficult to trap in the FD the excess charges generated in the PD, and to thereby expand the dynamic range.
The present invention has been made in view of the above described problem. An aspect of the present invention is to expand a dynamic range in a pixel structure in which an accumulating unit for temporarily accumulating charges accumulated in photoelectric conversion devices is shared by a plurality of pixels.